In general terms, an Erasable, Programmable, Read-Only, Memory (EPROM) package comprises:
(a) an insulating board having a chip mounting portion and having conductive interconnecting patterns thereon; one end of each of those patterns being adjacent to the chip mounting portion; PA1 (b) an EPROM chip mounted on the chip mounting portion; the chip having a plurality of electrodes on a surface thereof; PA1 (c) means for connecting said electrodes to the one end of each of the patterns; and PA1 (d) a covering insulating board containing a window composed of an ultraviolet transmitting glass.
This invention is directed to the production of a family of ultraviolet transmitting glass compositions expressly designed for use as windows in EPROM chips. The following four criteria have been determined to be quite desirable in glasses designed for that application:
First, the glasses must manifest coefficients of thermal expansion (0.degree.-300.degree. C.) between 46-52.times.10.sup.-7 /.degree.C. to insure sound sealing with Al.sub.2 O.sub.3 which is commonly used as the insulating material in EPROM packages;
Second, the glasses must demonstrate transmittances of at least 80% in thicknesses of 1 mm at a wavelength of 254 nm;
Third, the glasses must possess softening points below 700.degree. C. to permit rapid sealing to Al.sub.2 O.sub.3 ;
Fourth, the glass compositions must be free of fluoride inasmuch as volatilization of fluorine during melting leads to difficulties in glass forming and is an environmental hazard.
The types of aluminas used by EPROM fabricators vary widely. Hence, the various aluminas have different microstructures and, as a result, different intrinsic strengths and thermal expansions. As an illustration, the coefficient of thermal expansion of commercially-marketed aluminas (0.degree.-300.degree. C.) can vary over the range of about 65-80.times.10.sup.-7 /.degree.C. Such substantial variation quite obviously affects both the integrity of a glass-alumina seal and the amount of stress generated in the alumina during cooling of the seal, given any single glass composition window.
In common sealing practice, an expansion mismatch between the components being sealed together of less than 500 parts per million (ppm) is generally interpreted as a good seal, while differentials of less than 1000 ppm can be resolved into sound seals in more extraordinary circumstances. In the fabrication of EPROM packages, experience repeatedly demonstrated that expansion differentials between the glass and alumina greater than about 500 ppm create excessive stresses on both components of a seal. In such seals the excessive stress developed in the alumina has been deemed to cause the alumina components of the EPROM package to fracture.
Another extremely significant circumstance which must be considered in designing materials for EPROM packages is the fact that such packages must survive very demanding thermal shock requirements. Hence, the higher thermal conductivity of alumina during heating, when compared to conventional glasses, causes it to expand at a more rapid rate than the glass, thereby hazarding a loss of hermeticity in a seal with glass. Accordingly, a thermal expansion differential between the glass and alumina no less than about 200 ppm has been considered essential.
Therefore, the primary objective of the present invention was to develop glasses suitable for fabrication as windows for EPROM chips which satisfy the four above-mentioned criteria and, in addition, exhibit a thermal expansion differential between about 200-500 ppm when sealed in alumina.